/Engine/Source/Runtime/AIModule/Classes/Blueprint/AIBlueprintHelperLibrary.h
The way it works exactly is if the actor passed in is a pawn, then the function retrieves pawn's controller cast to AIController. Otherwise the function returns actor cast to AIController.
static [AAIController](API\Runtime\AIModule\AAIController) * GetAIController ( [AActor](API\Runtime\Engine\GameFramework\AActor) * ControlledActor )